Twenty Twelve - videos break outside content in responsive layout

Description

We need to account for video responsiveness within the new Twenty Twelve theme. I would recommend adding fitvids.js to the theme to be able to accurately calculate videos with different sizes. A pure CSS solution would involve setting a fixed aspect ratio, which could cause undesirable results for videos that have a non-widescreen aspect ratio.

Fitvids.js (or a similar PHP solution) is perfect for this problem because it looks for iframes with vimeo.com in the src attribute. CSS can achieve a similar effect, but it's impossible to scale the height based on the width.